These problems should be considered when adjusting the switching frequency
Time:2022-08-27
Views:1816
The switching mode power supply adopts fixed, adjustable or synchronous frequency with external clock for switching. The switching frequency value determines the overall dimensions of the capacitor and inductor of the power supply, and therefore also determines its cost. To design small low-cost circuits, designers began to use higher switching frequencies.
According to its data book specifications, the oscillator built in the switching regulator IC is usually used for a very wide frequency range. For example, the monolithic adp2386 Buck Converter IC can ensure that its switching frequency is within ± 10% of the set value. Other commonly used switching regulator ICs are specified as ± 20% or higher of the set value. Since the switching frequency of adp2386 has a variation range of ± 10%, in extreme cases, adp2386 uses RT to set the switching frequency to 600 kHz, which can switch at 540 kHz and 660 kHz
Figure 1. The switching frequency of adp2386 buck converter is set by resistor RT.
When designing the circuit, it must be considered that the switching frequency may vary by 20%. Since the peak current flowing through the inductor will vary with the actual switching frequency, the current ripple of the inductor will directly affect the output voltage ripple.
Fig. 2 shows the effect of switching frequency on inductance current ripple. In the figure, the nominal switching frequency of 600 kHz is shown in blue. The minimum (540 kHz) switching frequency is shown in purple and the maximum (660 kHz) switching frequency is shown in green. At the nominal setting frequency of 600 kHz, when the regulator is switched at 540 kHz, it can be seen that the peak to peak ripple current is 1.27 a. However, under the same frequency setting of 600 kHz, the switching regulator can also switch at 660 kHz, and the corresponding ripple current is 1.05 a. In this example, the difference in the coil current ripple of 220 Ma may be caused by the change in the switching frequency of different components in the circuit. This has exceeded the entire allowable temperature range
Figure 2. Peak to peak coil current ripple affected by switching frequency change
This factor must be taken into account in setting the current limit value of the switching regulator. The peak current must be low enough to ensure that no existing overcurrent protection is activated during normal operation.
Please note that this example does not consider all other possible change factors, such as inductance and capacitance changes.
Fig. 3 shows the corresponding output voltage ripple values for different current ripple changes. The circuit is designed to generate 4.41 MV ripple voltage when the switching frequency is 600 kHz. At 540 kHz switching frequency, the ripple voltage is 5.45 MV; The ripple voltage is 3.66 MV at 660 kHz switching frequency.
Figure 3. Output voltage ripple change caused by switching frequency change in switching mode regulator IC
In this example, the only variable considered is the change in switching frequency within the allowable temperature range. In practical applications, there may be many other variables, such as the actual value changes of inductance and capacitance. These are also affected by the operating temperature. However, we can also assume that in most cases, the actual change in switching frequency will not reach the limit of ± 10%. Generally, the switching frequency will vary around the typical value in the middle of the specified range. In order to systematically consider all dynamic variables in the power supply, we can find the answer through Monte Carlo analysis. The changes of different components and variable parameters are weighted according to the probability of occurrence and are correlated with each other. Use ADI‘s free LTSpice ® The simulation software can conduct Monte Carlo analysis.
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